Radio direction finder



Patented Mar. 11, 1947 NITED ST trice RADIO DIRECTION FBWDER David G. C. Luck, Merchantville, N. J'., assigner,- to Radio Corporation of America, a corporation of Delaware This invention relates to radio direction nnders andi, more particularly to automatic gain control systems for direct indicating direction finders.

A direct indicating direction nder is required to respond rapidly to signals over a wide range of unpredictable intensities. The effectiveness of manual gain control is limited by the skill of the operator, who is on occasion too slow.

During deep fades, caused by unfavorable polarzation` or by interference between signal rays following different paths, any direction finder is likely to give erroneous bearings. Ordinary automatic gain control acts not only to equalize signals of different strengths, but also to smooth out fading of any one signal, thus masking these unfavorable conditions.

Accordingly it is an object of this invention to provide anA improved method of and means forA automatic gain control to meet the special needs of direct indicating direction finders.

A further object is to provide 'improved means for warning against, or preventing, the reading of bearings with a direction finder when the output of its receiver is less than a predetermined fraction of the,maximum permitted by the automatic gain control.

These and other and incidental objects will becomerapparent to those skilledV in the art 'upon consideration of the following description, with reference to the accompanying drawing, in which Figure 1 is a schematic block diagram of a direction finder embodying the invention, and Figure 2 is a circuit diagram of a gain control circuit.

The illustrated direction finder is of the type described in U. S. Patent 2,208,378, filed June 3l), 1'938, by David G. C. Luck, and entitled Continuously indicating radio compass. An antenna, ar ray l of the Adcock type is connected through modulating means 3 to a radio receiver 1, which in this case is shown separately from its detector 9. The radio receiver 3 is connected through the detector 9 to a phase meter bearing indicator, or equivalent device Il. The modulators 3 are controlled by a local oscillator or modulation generator 5, which is also connected to the phase meter Il. The phase meter may be calibrated in terms of bearing, and will indicate directly the azimuth of arrival of waves at the antennas.

The gain control system I is connected to the radio receiver 1 ahead of its detector 9, and arranged to supply a unidirectional voltage to the controlled circuits of the receiver l, in conventional fashion. A rectifier l1 is also connected to the receiver 1 and through a threshold device lil to a relay 2| arranged to control a warning device such as a lamp 23.

The adjustment and operation of the system is.

to variations in theoutputof the receiver 1. TheV voltage at the output of the network ccntrolsthe gain of the receiver accordingly. A suitable gain.

control network is illustrated inFig. 2'comprising a diode rectifier 25 connectedto the output circuit of the receiver l and through a resistor 2l to a capacitor landa shunt resistor 3l. The build up and-decay times in the control voltage output `of the circuit in response to variationsin the radio or intermediate frequency input may be controlled separately because the resistance of the diode is` relatively low for one direction of cur- Y rent flow and substantially innite for the other. The rate at which the condenser Z9 is charged upon a, given increaseof receiver output depends primarily on the value of the resistor 21 and the internal resistance of the receiver output circuit. The rate of discharge of the condenser 29 depends upon the capacity of the condenser and upon the value of the resistor 3|, because the diode 25 acts as an open circuit to the flow of the discharge current. The value of the resistor 3l is preferably much larger than the resistance of the other parts of the circuit, and may be made variable as indicated to enable manual control of the recovery time of the gain control.

A direct indicating direction finder should be arranged to be very sensitive in the absence of received signals, but to have its gain automatically cut down very rapidly when a signal is received. Thus, it will normally be in condition to indicate the bearing of even a very Weak signal, but will never be overloaded by a strong signal. It should be prevented from following fading cycles by providing for comparatively slow restoration of gain upon decrease of signal. The rate of recovery must bea, compromise between the factors of complete masking of fading conditions and a long period of low sensitivity following cessation of a strong signal. The recovery rate of the network l5 may be made manually adjustable, to enable the operator to match the fading conditions present at any time, since the duration of the fading cycle is Widely variable.

With an automatic gain control functioning as described above, and some means of indicating the receiver output level, a moderately skilled operator can easily tell when bearing indications should not be relied upon because of adverse fading conditions. However, it may be undesirable to rely too heavily on the skill and judgment of the operator. The circuit comprising the rectier II, threshold device l9,`relay 2| and lamp 23 automatically warns against the reading of bearings when the receiver output is less than a predetermined fraction of the maximum output permitted by the automatic gain control.

The rectier l1 derives a control voltage which follows in amplitude any variations in the output level of the receiver 1. This voltage is applied to the threshold device I9 which is adjusted so that when the control voltage exceeds, for example, one third its possible maximum, the relay 2| is actuated by the output of the device I9 to extinguish the warning lamp 23. This provides a disjunctive, or on and off indication of whether the signal level is above or below the minimum required for reliable bearing indications.

Thus the invention has been described as a radio direction finder incorporating an automatic gain control system arranged to meet the peculiar needs of direct indicating direction iinders by acting rapidly on increase and slowly on decrease of signal strength. To minimize the skill and judgment required of the operator a warning device is provided for indicating when fading conditions are such as to result in unreliable bearing indications.

I claim as my invention: I

1. A radio direction finder including a receiver provided with automatic gain control means responsive with relatively slight delay to increase of signal and with relatively great delay to decrease of signal, an auxiliary rectifier connected to said receiver and responsive with negligible delay to the amplitude of the output thereof to provide va control voltage having a predetermined relationship to said amplitude, a voltage threshold device connected to said rectifier and responsive to said control voltage to provide output only when said control voltage exceeds a 4 predetermined magnitude, and indicator means connected to said threshold device.

2. A direction inder comprising a radio receiver, automatic gain control means for said receiver including a timing circuit arranged to provide rapid response to increase in the amplitude of the output of said receiver and relatively slow response to decrease in the amplitude of said output, means connected to said receiver and responsive with negligible delay to the output thereof to provide a control voltage having a predetermined relationship to the magnitude of said output, an indicator device and a threshold device, responsive to said control voltage to provide output only when said control voltage exceeds a predetermined magnitude, and connected to said indicator device to provide disjunctive actuation thereof in response to variations in the output of said receiver about a predetermined level.

3. A radio direction iinder including a radio receiver with automatic gain control, means responsive to the amplitude of the output of said receiver to provide a control voltage proportional to said amplitude, and indicator, and a threshold device connected to said indicator and controlled by said control voltage to provide disjunctive operation of said indicator.

DAVID G. C. LUCK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,208,733 Roberts July 23, 1940 2,223,995 Kotowski et al Dec. 3, 1940 2,318,338 Simon et al. May 4, 1943 1,485,773 Espenschied Mar. 4, 1924 2,108,899 Peterson Feb. 22, 1938 2,226,366 Braden Dec. 24, 1940 FOREIGN PATENTS Number Country Date 472,891 British Sept. 29, 1937 524,526 British Aug. 8, 1940 

